This invention relates to the field of spirometers. Spriometers have been known for some time as a means of measuring the volume of air exhaled by a patient. One type of spirometer is disclosed in U.S. Pat. No. 4,182,175, the disclosure of which is incorporated herein by reference.
In spirometers such as those described in the above-cited patent, there is provided a stator, and a rotor disposed within the stator. The stator has slots allowing air to enter from the periphery, and to cause the rotor to rotate. In spirometers of the type described, the flow of air sets up a vortex within the stator. The turbine rotor rides around with this vortex, and the speed, in rotations per minute (rpm) of the vortex is linearly proportional to the flow over a fairly wide range.
The problem with turbine systems of this kind is that the vortex generally continues to rotate somewhat after the air flow stops. Since the vortex is typically spinning at 30,000 to 60,000 rpm, there is considerable energy to dissipate. Without any flow, generated by the exhalation of the patient, to move the vortex into the exit tube of the spirometer, there is not much friction to stop the spinning. Thus, the rotor generally continues to turn somewhat after the patient ceases to exhale. The rotor is said to "coast." In the prior art, the only means of eliminating coasting has been to rely on friction of the mechanical gear train. However, when either a low friction gear train or an electronic or radiation beam type of rotation counter and/or flow measuring device is used, this coasting has made it impractical, in the prior art, to derive a really accurate reading of volume of air flow based on the number of turns of the rotor.
The present invention provides a structure which eliminates coasting, and thus makes it possible to measure the volume of air flow directly from the rotor. Since it is then practical to use a low-friction drive train or friction-free measuring device, it is possible to have an extremely sensitive spirometer, which responds to extremely low air flows.